The present invention relates to photodetectors, and in particular to Metal-Semiconductor-Metal (MSM) photodetectors.
Metal-Semiconductor-Metal (MSM) detectors are simple planar devices that are used in high-speed photodetectors in fiber-optic data communication with bandwidth extending above 25 GHz. Prior art MSM detectors have consisted of interdigitated planar metal electrode pairs 2a, 2b on high-mobility semiconductor surfaces 1, as illustrated in the prior art structure of FIGS. 1A and 1B. The neighboring metal electrodes form a back-to-back Schottky-diode pair. Under both possible biasing polarities, one of the Schottky diodes is always reverse biased, therefore in the absence of light only a very small xe2x80x9cdark-currentxe2x80x9d can flow as a result of thermal excitation and tunneling of carriers at the metal-semiconductor interface. When these prior art devices are illuminated with photons of sufficient energy, electron-hole pairs (shown as xe2x88x92/+ and exe2x88x92/h+ in FIGS. 1A, 1B) are generated in the semiconductor 1 and the charge carriers are swept to the metal electrodes 2a, 2b under a few Volts bias. The photocurrent is proportional to the flux of photons impacting the uncovered border area 1a of semiconductor 1 within the total detector area.
For convenience, the xe2x80x9ctotal device areaxe2x80x9d may defined as the minimum area definable by a simple closed convex curve chosen so that all of the metal of the photodetector lies within its interior (the metal would not include any wiring necessary to connect the device to an external circuit).
For discussion hereinbelow, we further define a xe2x80x9cborder areaxe2x80x9d 1a as the area of uncovered semiconductor 1 that is between the two metal electrodes 2a, 2b and within the total device area. The photogenerated charge by a light pulse is proportional to the number of photons in the pulse.
Two general points can be made about the performance of such prior art MSM photodetectors:
First, the magnitude of the photo-induced charge generated by a unit pulse of photons impacting the detector is directly proportional to the internal quantum yield (i.e. the probability that photoexcitation of electron-hole pairs takes place within a certain depth in the semiconductor) and magnitude of the border area of the detector.
Second, the speed of these prior art MSM photodetector is determined by the carrier mobility in the semiconductor, the distance the carriers have to travel from the point of generation to the electrodes; as well as, by the capacitance of the interdigitated metal electrodes
For reasons of higher carrier mobility, quantum-yield and dark-current requirements, the more frequently used semiconductor substrates are III-V compound semiconductors (GaAs, AlGaAS, AlInAs, InGaPxe2x80x94InP, etc.) with a variety of Schottky-contact metals (Pt, Ti, Al, Au, Cu, W, . . . etc.). To achieve high speed, i.e. shorter detector response time, the width of the electrode fingers and the finger separations are on a micron to sub-micron scale. As can easily be appreciated, the requirements for sensitivity (a large photogenerated charge per unit light pulse) and speed are in conflict. Sensitivity is improved by increasing the border area, while speed is increased by moving the electrodes close together, which inherently decreases border area. Very thin, highly interdigitated electrodes, an apparent solution to this conflict in requirements, results in an increase in electrode resistance and total device capacitance, which decrease the device speed.
In data-communication the real physical source of transferred data is almost exclusively a silicon (CMOS) chip. Similarly, the transferred data is received for processing, storage or routing nearly exclusively by a CMOS device. Silicon-based optoelectronic devices, that can be monolithically integrated with these CMOS chips, would be therefore highly preferred. Prior art photodetectors, all III-V based devices, have a high packaging expense component when attached to the CMOS devices, and economy would suggest to replace the Ill-V semiconductors in these photodetectors with silicon. However silicon has a lower carrier mobility than the III-V semiconductors. This lower mobility has been perceived to cause unacceptable speed limitations, and has led to the neglect of silicon as semiconductor substrate of choice for high-speed photodetectors.
The present invention broadly provides a photodetector for detecting electromagnetic radiation which is incident thereon, the photodetector comprising:
a) a substrate comprising a semiconductor material characterized by an electron energy bandgap, the aforesaid substrate having a surface, and
b) a pair of electrodes disposed upon said surface to define therebetween a border area of the aforesaid surface, a first electrode of the aforesaid pair comprising a layer of metal disposed upon the aforesaid surface and exposed to the aforesaid incident radiation, the aforesaid first electrode covering an area of the aforesaid surface which is larger than the aforesaid border area, the aforesaid metal being characterized by a Fermi level which is within said bandgap.
Preferably, the semiconductor material is selected from silicon and an alloy of silicon and germanium. Moreover, the substrate may comprise a semiconductor layer deposited upon an insulator.
According to a preferred embodiment, the first electrode covers an area at least three times as large as said border area. Moreover, it is preferred that the metal comprise a member selected from the group consisting of Al, Co, Cr, Fe, Mn, Nb, Ru, Ta, Ti, V, W, and Zr.
According to another embodiment, the photodetector comprises a layer of insulator material disposed between the aforesaid surface of the semiconductor substrate and the aforesaid pair of electrodes, wherein the insulating layer is sufficiently insulating to substantially eliminate dark current between the pair of electrodes while permitting the flow of photogenerated current therebetween. For this purpose, the aforesaid layer of insulator material may be characterized by an electron energy bandgap of at least 3 ev.
Preferably, the layer of insulator material comprises SiO2 and has a thickness of less that 50 Angstroms.
According to another embodiment, the aforesaid first electrode exhibits an anti-reflection treatment. This anti-reflection treatment may be effected by, for example, applying an anti-reflection coating (e.g. one that comprises SiN) or may alternatively comprise a roughening of an exposed surface of the aforesaid layer of metal.
Preferably, the energy difference between the Fermi level of the aforesaid metal of the first electrode and the midpoint of the semiconductor bandgap is no more than 20% of said semiconductor bandgap. One choice of metal is tungsten, which may be deposited by chemical vapor deposition from tungsten hexacarbonyl.
According to a preferred embodiment of the photodetector, the aforesaid layer of the first electrode is adapted to be exposed to the incident radiation over a first exposure area thereof, the aforesaid border area is adapted to be exposed to said incident radiation over a second exposure area and wherein said first exposure area is substantially larger than said second exposure area.